ES2583834T3 - Method for purification of N-alkyl pyrrolidones - Google Patents

Abstract

Method for the purification of N-alkyl-pyrrolidones, which, due to a use that has already occurred, contain at least one of the contaminants of formulas I or II ** Formula ** in which R represents hydrogen or a C1- alkyl group C20, characterized in that -a basic compound is added to the N-alkyl-pyrrolidone to be purified and no more than 20 minutes after the addition of the basic compound, the temperature is at least 80 ° C and - is separated by distillation of the N-alkylpyrrolidone from the mixture obtained.

Description

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DESCRIPTION

Method for purification of N-alkyl pyrrolidones

The present invention relates to a method for the purification of N-alkyl pyrrolidones, which by an already occurred use contain at least one of the contaminants of formulas I or II

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5 in which R represents hydrogen or a C1 to C20 alkyl group.

This method is characterized in that - a basic compound is added to the N-alkyl pyrrolidone to be purified and no more than 20 minutes after the addition of the basic compound, the temperature of the mixture is at least 80 ° C and 10-N-alkyl pyrrolidone is distilled off from the mixture obtained.

In addition, the present invention relates to a method for the production, purification and new use of N-alkyl pyrrolidones, which is characterized in that a) a polymer is dissolved in an N-alkyl pyrrolidone, b) with this polymer electrodes are produced of lithium ion accumulators and in this regard an N-alkyl is recovered

Contaminated pyrrolidone, which contains at least one of the contaminants of formulas I or II

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where R in formulas I and II represents hydrogen or a C1-to C20 alkyl group,

c) a basic compound is added to the recovered N-alkyl pyrrolidone and not more than 20 minutes after the addition of the basic compound, the temperature of the mixture rises to at least 80 ° C,

D) by distillation the N-alkyl pyrrolidone is separated from the mixture obtained, and

e) is used again as a solvent for the production of electrodes for lithium ion accumulators.

Polymers are often used for the production of lithium ion accumulators, for example as a polymer electrolyte or for the production of electrodes. For example, for the production of electrodes the use of polyvinylidene fluoride is common.

25 To this end, polyvinylidene fluoride is commonly dissolved in an organic solvent and by adding materials that store lithium and, where appropriate, conductive substances, a suspension is produced. By coating a support, for example a metal support, with this suspension and subsequent solvent removal, the electrodes are obtained. N-alkyl pyrrolidones, in particular N-methylpyrrolidone, have proven to be suitable solvents.

30 Due to the previous use the solvent can be contaminated. Contaminants are incorporated through the coating process, secondary products of the solvent used also arise, due to the high temperatures required and / or the presence of oxidizing compounds. With N-methyl-pyrrolidone (NMP) they arise by unwanted side reactions, composed of the formulas above I and II, in which R represents an atom of H. These have boiling points similar to that of NMP and therefore separate Hardly by distillation of the NMP.

35 For the production of long-life lithium-ion accumulators, only the use of solvents with high purity and anhydrous substances is allowed. Solvents used once can only be used again when they have been purified and the high requirements of purity and absence of water are satisfied.

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From JP 11071346 and JP 10310795 the purification of NMP is known which is used as a solvent for polyvinylidene fluoride, by the addition of acidic compounds, in particular by contact with acidic substances.

WO 2010/057917 describes the purification of NMP that was used as a solvent for polyvinylidene fluoride and for the production of lithium ion accumulators. Purification occurs by the addition of activated carbon, hence it is an adsorption purification.

The addition of alkaline or alkaline earth hydroxides to NMP has also been described. Thus, from JP 2004284958 and JP 2007099690, the distillation of NMP in the presence of alkaline or alkaline earth hydroxides is known, to thereby prevent the formation of peroxides and stabilize the NMP.

In US 4965370, JP 2001354769 and JP 11349566, the purification of NMP is described, which was used in the production of polyarylene sulfide; By adding alkaline or alkaline earth hydroxide, contaminants can be removed in each case from the polyarylene sulfide process and the NMP is used again for these processes.

JP 03014559 states the removal of N-methyl succinimide and phenol from NMP by the addition of metal hydroxides or metal carbonates. Also here it is an NMP of the polyarylene sulfide process.

WO 03/053924 describes a method for the production of NMP from gamma-butyrolactone and methylamine. Here hydroxides are used metal, to remove excess gamma-butyrolactone by salt formation.

An objective of the present invention was a method for the purification of N-alkyl pyrrolidone already used, in

In particular NMP, which allows the new use of N-alkyl pyrrolidones for any application, in particular for the production of lithium ion accumulators. The method should be as easy and effective as possible. By means of the method, contaminants from formulas I and II incorporated or arising should be removed as completely as possible.

Therefore, the method defined above was found.

The method according to the invention is a method for the purification of N-alkyl pyrrolidones, which were already used at least once. Examples may be N-alkyl pyrrolidones that had been used as a solvent.

Preferably they are N-alkyl pyrrolidones of the formula A

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In which R represents hydrogen or a C1-to C20 alkyl group.

In particular R in formula A represents hydrogen or a C1-to C3 alkyl group, particularly preferably R represents hydrogen or a methyl group. If R represents hydrogen, it is an N-methyl-pyrrolidone (NMP), if R represents a methyl group, it is an N-ethylpyrrolidone (NEP). Particularly preferred, they are NMP or NEP. Very particularly preferred, it is NMP.

35 Due to a use that has already occurred, N-alkylpyrrolidone has at least one of the contaminants in formulas I

or II in which R represents hydrogen or a C1-to C20 alkyl group.

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The compounds of formulas I and II are isomers that frequently occur in admixture. That is why the N-alkylpyrrolidone to be purified can contain both compounds. Preferably R is a hydrogen or a C1-to C4 alkyl group.

5 When the N-alkyl pyrrolidone to be purified is NMP, R in formulas I and II represents hydrogen.

When the N-alkyl pyrrolidone to be purified is NEP, R in formulas I and II represents a methyl group.

The content of compounds of formulas I and II in the N-alkyl pyrrolidone may in particular be 0.0005 to 1 part by weight over 100 parts by weight N-alkyl pyrrolidone. In the present method, in particular, an N-alkylpyrrolidone is used, which has a content of compounds of formulas I and II of in total 0.0005 to 0.5, so

Particularly preferred from 0.0005 to 0.1 parts by weight over 100 parts by weight of N-alkyl pyrrolidone.

Apart from the compounds of formulas I and II, the N-alkyl pyrrolidone to be purified may contain other organic compounds as contaminants.

Other organic compounds, which may be present as contaminants, are for example those of the following formulas

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in which R represents in each case hydrogen or a C1-C20 alkyl group.

Apart from the compounds of the formulas I, II and given the case III to XII, the N-alkylpyrrolidone to be purified may contain other organic compounds, which originate already in the production process of the N-alkyl20 pyrrolidone and not They are counted in pollutants. Apart from gamma-butyrolactone and the amine used for the production of N-alkyl pyrrolidone (methylamine in the case of NMP), these other organic compounds are derived from N-alkyl pyrrolidones, which in the carbon atoms of ring are substituted with alkyl radicals. Preferred alkyl radicals are methyl and ethyl groups and can be present in all carbon atoms of the ring individually or as a mixture. The content of these substituted N-alkyl pyrrolidones is in

Generally less than 1 part by weight, in particular less than 0.5 part by weight over 100 parts by weight N-alkyl pyrrolidone. The gamma-butyrolactone content is generally less than 0.1 parts by weight, in particular less than 0.05 parts by weight over 100 parts by weight of N-alkyl pyrrolidone. The amine content used for the production of N-alkyl pyrrolidone is generally less than 0.005 parts by weight, in particular less than 0.003 parts by weight over 100 parts by weight N-alkyl pyrrolidone.

The N-alkyl pyrrolidone to be purified contains about 100 parts by weight of N-alkyl pyrrolidone in general in total 0.0005 to 5 parts by weight, in particular 0.0005 to 2 parts by weight of compounds organic formulas I to XI, as contaminants.

The N-alkyl pyrrolidone to be purified may also contain water or be present in admixture with water. Through the above uses, greater amounts of water may have been incorporated.

Therefore, the N-alkyl pyrrolidone to be purified may be composed as follows of 100 parts by weight of N-alkyl pyrrolidone,

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in total 0.0005 to 5 parts by weight of organic compounds of formulas I to XII, as contaminants and

0.01 to 200 parts by weight of water.

The N-alkyl pyrrolidone to be purified may be composed in particular as follows of

5 100 parts by weight of N-alkyl pyrrolidone

in total 0.0005 to 2.5 parts by weight of organic compounds of formulas I to XII as contaminants and

0.1 to 100 parts by weight of water.

To the N-alkyl pyrrolidone to be purified is added a basic compound. Under the concept of basic compound, mixtures of different basic compounds are also understood below.

The basic compound is preferably a compound that in an amount of 5 parts by weight in 100 parts by weight of water (at 20 ° C, 1 bar) causes a pH value of at least 8, particularly preferably of at least 9. In the preceding definition it is not necessary that the 5 parts by weight of the basic compound be completely dissolved in water, it is only decisive that after the addition of the basic compound, the pH increase occurs.

The basic compound may be an organic or inorganic compound. For example, alkali and alkaline earth hydroxide, alcoholates, carbonates, carboxylates, complex hydrides, ammonia or amines come into consideration. Mentioned are for example LiOH, NaOH, KOH, Mg (OH) 2, Ca (OH) 2, Ca (HCO3) 2, CaCO3, sodium oxalate, sodium borohydride, lithium aluminum hydride, sodium methylate, ethanolate sodium, potassium methylate, potassium ethanolate and alkylamines corresponding to the N-alkyl substituted pyrrolidone.

Particularly preferred are inorganic compounds, in particular bases containing alkali metals or bases containing alkaline earth metals.

Very particularly preferably, the basic compounds used are NaOH, KOH, sodium methylate or potassium methylate.

In a particularly preferred embodiment, it is sodium hydroxide.

The basic compound can be added to the N-alkyl pyrrolidone to be purified, as such or as a solution. If the basic compound is added in solution, then the solvent may be for example water or an organic solvent, for example also the corresponding N-alkyl pyrrolidone or mixture of the corresponding Nalkylpyrrolidones with water.

The basic compound is preferably used in amounts of 0.01 to 10 parts by weight over 100 parts in

30 weight of the N-alkyl pyrrolidone to be purified (sum of N-alkyl pyrrolidone and all the organic components present there, as contaminants and derivatives). Particularly preferred are used in amounts of 0.01 to 1 and very particularly preferred in amounts of 0.01 to 0.5, in particular 0.05 to 0.5 parts by weight based on 100 parts by weight of the N-alkyl pyrrolidone to be purified.

No more than 20 minutes, preferably no more than 10 minutes, particularly preferably no more than 5

35 minutes, very particularly preferably no more than 2 minutes after the addition of the basic compound, the temperature of the mixture obtained from the N-alkyl pyrrolidone to be purified and the basic compound, is at least 80 ° C.

Preferably the temperature is at least 100 ° C.

The preceding temperature can be easily adjusted, by

- Joint addition of the basic compound and the N-alkyl pyrrolidone to be purified and rapid heating of the obtained mixture, thereby reaching the necessary temperature in the time described previously (alternative 1) or

- preheating of the N-alkyl pyrrolidone to be purified, so that after the addition of the basic compound the desired temperature is reached immediately (alternative 2).

A particular embodiment of alternative 2 is the addition of the basic compound directly to the distillation, by which the purified N-alkyl pyrrolidone is separated.

Without raising the preceding temperature, solid matter can easily precipitate. In this regard, it is a disadvantage that, for example, pipes and valves can be occluded, which may make it necessary to stop the installation and involve a corresponding cleaning cost.

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The N-alkyl pyrrolidone is separated from the mixture by distillation. The purified N-alkyl pyrrolidone is preferably removed from the head of the column.

5 The distillation is preferably carried out at a temperature of 50 to 350 ° C, particularly with reference to 100 to 250 and a pressure of 0.3 to 5 bar.

As columns for distillation, columns known to experts are suitable. Packed columns, floor columns with sieve floors, columns with dual flow floors, bell floor columns are preferred

or rectification columns equipped with valve floors, separating wall columns or evaporators of falling film or thin layer.

Preferably distillation is performed in the absence of oxygen. It is understood herein by the absence of oxygen, that the volume fraction of oxygen is less than 0.1%, in particular less than 0.01 and very particularly less than 0.001%, based on the total volume of the distillation column.

Preferably, the addition of the basic compound to the N-alkyl pyrrolidone is already carried out under an inert gas atmosphere,

15 so that during the whole method the oxygen content is reduced. Preferably, the whole method is executed in the absence of oxygen, in which the absence of oxygen is understood to mean an oxygen fraction of less than 0.1%, in particular less than 0.01 and very particularly less than 0.001%, based on the volume total of the respective part of the apparatus used (container, distillation columns).

The used apparatuses are preferably constructed of stainless steel, likewise all the pipes and 20 components of the column.

The method can be executed continuously or discontinuously.

In the discontinuous execution, the basic compound can be added to a pre-existing amount of the N-alkylpyrrolidone to be purified and the mixture obtained be distilled in portions.

Preferably the method is executed continuously.

For this purpose, the starting materials are continuously combined, that is to say the basic compound and the N-alkyl pyrrolidone to be purified. For example, they can be conducted continuously to a storage container and mixed there; Distillation can be fed continuously from this storage vessel.

The basic compound and the N-alkyl pyrrolidone to be purified can be supplied as separate streams directly and continuously to distillation.

The N-alkyl pyrrolidone to be purified is preferably supplied at the exit part of the column. The basic compound can be supplied as a pure substance or as a solution to any floor of the distillation column.

Distillation can occur in one or several stages, that is to say in one or several columns.

For continuous distillation in one stage, for example a separation wall column is also suitable.

In a preferred embodiment, continuous distillation is carried out in two stages.

For this, the N-alkylpyrrolidone to be purified (preferably NMP to be purified) and the basic compound (preferably NaOH) can be fed in mixture or separately to the first distillation column 40. In the first distillation column, preferably low boiling substances such as methylamine, in particular water, at a bottom temperature of 150 to 250 ° C, in particular 180 to 250 ° C, and a pressure of 0 are separated from the head , 1 to 5 bar, in particular 0.3 to 3 bar. In the second distillation column the purification distillation of the bottom of the first distillation column can then occur, under reduced pressure, for example at a bottom temperature of 80 to 180 ° C, in particular 100 to 160 ° C and a pressure of 0.005 to 0.5 bar,

45 in particular 0.01 to 0.3 bar. The purified N-alkylpyrrolidone can be separated from the second column as lateral removal and continuously removed from the bottom, which contains contaminants and the basic compound or its reaction products.

A particular embodiment of the method is to perform the distillation of the N-alkyl pyrrolidone to be purified, in admixture with additional N-alkyl pyrrolidone from the production process.

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N-alkyl pyrrolidones, in particular NMP, can be obtained by reacting gamma-butyrolactone with the corresponding amine (methylamine in the case of NMP). In this reaction, a product stream of N-alkyl pyrrolidone (NMP), water, starting materials that did not react (gamma-butyrolactone and methylamine) is obtained and, if necessary, by-products. This product stream is conditioned by distillation. N-alkyl

The pyrrolidone to be purified can be mixed with this product stream in any quantity. The Nalkylpyrrolidone to be purified may contain the basic product already before the combination with the product stream. Alternatively, it is also naturally possible to add the basic compound to the product stream, so that the basic compound is fed to the N-alkyl pyrrolidone to be purified, and by mixing with the product stream.

10 After distillation, the purified N-alkylpyrrolidone contains about 100 parts by weight N-alkyl pyrrolidone

-in total less than 0.5 parts by weight of contaminants III -XII

-Less than 0.05 parts by weight of water and

- in total less than 0.01 parts by weight of compounds of formulas I and II

In particular, purified N-alkyl pyrrolidone contains, over 100 parts by weight of N-alkyl pyrrolidone

15 -in total less than 0.1 parts by weight of contaminants III -XII

-Less than 0.03 parts by weight of water and

- in total less than 0.0005 parts by weight of compounds of formulas I and II

The purified N-alkyl pyrrolidone contains in particular less than 5% parts by weight, preferably less than 1% part by weight, particularly preferably less than 0.1% parts by weight of the total amount.

20 originally present in the contaminated N-alkyl pyrrolidone, of the compounds of formulas I and II.

Due to its high purity, the purified N-alkyl pyrrolidone obtained is suitable for the new production of electrodes for lithium accumulators.

Examples

The following examples should serve to clarify the invention. The contents of contaminants indicated in the

25 examples are determined by gas chromatography (CG apparatus HP6890, FID detector, nitrogen support gas with 1.0 mL / min (constant flow); cleavage ratio 1:50; RTX-1 column, 30 m, 0.32 mm, 1.0 µm film; temperature program starts at 80 ° C, then 5 ° C / min up to 140 ° C, then 5 ° C / min up to 200 ° C and 10 min isothermal, then 10 ° C / min up to 340 ° C and 8 min isothermal). Water values are determined by Karl-Fischer titration. NMP were used as well as the corresponding contaminants in the

30 quantities indicated in the examples. All stages of work occurred in clean appliances under nitrogen atmosphere, by GC the absence of foreign compounds was controlled before the start of the test.

Comparison Example 1

At an NMP contaminated with 0.5% water and 0.031% contaminants of formulas I -XII (contaminants of formulas I and II, 0.004%; contaminants of formulas III, 0.017%; contaminants of formulas IV -VI , 35.003%; contaminants of formulas VII -IX, 0.002%; contaminants of formulas X -XII, 0.005%) 0.1% NaOH (calculated as pure NaOH, used as 30% aqueous solution) was added and It was pumped to a tank container. After a residence time of 4 h at a temperature of 15 ° C, approximately 100 kg of a deposit precipitated, for an initial amount of 23.5 t of NMP. For the next two-stage distillation the contents of the tank were moved by means of a lifting tube continuously in an amount of 500 L / h to the first 40 valve floor column (40 floors, feed on the 21st floor) and stopped the feed after filling 23.3 t, so that the solid matter remained in the tank container. Distillation occurred at bottom temperatures between 180 and 185 ° C, a column head pressure of 530 mbar and a return-to-reduction ratio of approximately 1 to 1. The bottom of the column was led to the second floor column of valve (40 floors, feed on the 21st floor) and was distilled at background temperatures between 165 and 170 ° C, a head pressure of

45 column of 220 mbar, the product was removed on the head. The sodium content in the feed to the second valve floor column was between 0.0005 and 0.001%, the distillate also exhibited the contaminants of formulas I and II in 0.002% and contaminants of formulas IV-VI in 0.001% and was free of the contaminants of formulas III, the distillation yield was 90% and the water content in the product was 0.005%.

50 Comparison example 2

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At an NMP contaminated with 1.0% water and 0.074% contaminants of formulas I -XII (contaminants of formulas I and II, 0.003%; contaminants of formulas III, 0.035%; contaminants of formulas IV -VI , 0.002%; contaminants of formulas VII -IX, 0.032%; contaminants of formulas X -XII, 0.002%) 0.1% NaOH (calculated as pure NaOH, used as 30% aqueous solution) was added and added pumped into a container. 5 Within a residence time of 6 h at a temperature of 25 ° C approximately 0.1 kg of a deposit precipitated, for an initial amount of 30 kg of NMP. For the next distillation, the contents of the container were transferred to the ampoule of a distillation apparatus and step fractionation was performed. The solid matter remained in the container. Distillation distilled in a column (Sulzer CY packing, 13 pulses every 0.63 m, 1 pulse every 0.30 m) with a return divider at bottom temperatures between 130 to 140 ° C and pressures 10 at the beginning of 1000 mbar ( separation of substances with low boiling point) and 90 mbar of head pressure at a return to reduction ratio of 50 to 1. A total of 30 distillation fractions were taken. In the first five fractions, which were obtained at head temperatures of 55-118 ° C, an excess of water (> 98%) was found. At 100 mbar and a head temperature of 122-126 ° C the following fractions 6-24 were obtained (together approximately 21 kg). The water content of these was below 0.1%, NMP in purities above

15 of 99.8% despite the contaminants already present in the loading of formulas I and II, 0.001% and formulas IV -VI, 0.001%. From fraction 25 (together approximately 4 kg) they could no longer be detected by analysis of unwanted contaminating GC of formulas I-XII, the distillation yield was 13% and the water content in the product was 0.005%.

Examples according to the invention

20 Example 1

To the use mixture of Comparison Example 1 was added continuously in a static mixer with a residence time of 30 seconds, for an addition rate of the contaminated NMP of 500 liters (L) / hour

(h) and a temperature of 15 ° C, 0.1% NaOH (calculated as pure NaOH, used as 30% aqueous solution). In contrast to Comparison Example 1, this addition was conducted directly to the first column of

25 valve floor (addition on the 21st floor) and there it was heated within 2 minutes to above 150 ° C. The formation of solid matter was not observed. Distillation occurred in two stages analogously to Comparison Example 1. The sodium content in the addition for the second valve floor column was between 0.05 and 0.06%, the distillation yield was 90%, by GC analysis the contaminants of formulas I-XII could not be detected in the product and the water content was 0.005%.

30 Example 2

The use mixture of Comparison Example 2 was combined with 0.1% NaOH (calculated as pure NaOH, used as a 30% aqueous solution) in a static mixer with a residence time of 30 seconds and heated in a preheater within 10 minutes up to 95 ° C, where solid formation was not observed. With an addition rate of 5 L / min the mixture was transferred to the ampoule of the distillation apparatus and fractionation was carried out directly in steps analogously to Comparison Example 2 (heating at 130 ° C within another 20 minutes). A total of 20 distillation fractions were taken. In the first five fractions, which were obtained at head temperatures of 55-118 ° C, excess water (> 98%) was found. At 100 mbar and a head temperature of 122-126 ° C the following fractions 6-20 were obtained (together approximately 25 kg). The water content of these was below 0.1%, NMP in purities above 99.8% and by

40 CG analyzes could no longer detect unwanted contaminants of formulas I -XII. The distillation yield was 83% and the product water content was 0.005%.

Example 3

The mixture of use of Comparison Example 2 was combined with 0.15% potassium methylate (briefly KOMe, calculated as pure KOMe, used as a 25% methanolic solution) in a static mixer with a residence time of 30 seconds and heated in a stable pressure preheater within 10 minutes at 95 ° C, where solid formation was not observed. With an addition rate of 10 L / min the mixture was transferred to the ampoule of a distillation apparatus and fractionation was carried out directly in steps analogously to Comparison Example 2 (heating at 130 ° C within another 20 minutes) . A total of 20 distillation fractions were taken. In the first five fractions, which were obtained at head temperatures of 55-118 ° C,

50 found excess water (> 98%) and a small fraction of methanol. At 100 mbar and a head temperature of 122-126 ° C the following fractions 6-20 were obtained (together approximately 25 kg). The water content of these was below 0.1%, NMP in purities above 99.8%, and unwanted contaminants of formulas I-XII or methanol could no longer be detected by GC analysis.

Example 4

The use mixture (500 milliliters) of Comparison Example 1 was heated in a steel autoclave with 1% monomethylamine (calculated as pure amine, used as a 40% aqueous solution) within 15 minutes at 100 ° C and then it was further heated at 200 ° C for 6 h, at which a pressure of approximately 20 bar was generated. The solid-free reaction discharge was then distilled, analogously to the conditions of Comparison Example 2, in a laboratory equipment (standard distillation construction with Vigreux column). Approximately 90% of NMP was obtained in the desired purity (monomethylamine content <0.002%), in which the predominant residual was contaminated only by very high water content (> 0.1%).

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Claims (10)

image 1 1. Method for the purification of N-alkyl pyrrolidones, which by an already occurred use contain at least one of the contaminants of formulas I or II image2 In which R represents hydrogen or a C1-to C20 alkyl group, characterized in that a basic compound is added to the N-alkyl pyrrolidone to be purified, and no more than 20 minutes after the addition of the basic compound, the temperature is at least 80 ° C and the N-alkyl pyrrolidone is distilled off from the mixture obtained. Method according to claim 1, characterized in that the N-alkyl pyrrolidone is N-methylpyrrolidone. 3. Method according to claim 1 or 2, characterized in that the basic compound is a compound, which in an amount of 5 parts by weight in 100 parts by weight of water (at 20 ° C, 1 bar) causes a pH value of at least 8. 4. Method according to one of claims 1 to 3, characterized in that the basic compound is an alkaline or alkaline earth hydroxide.

5.

Method according to one of claims 1 to 4, characterized in that the basic compound is added in amounts of 0.01 to 10 parts by weight based on 100 parts by weight of N-alkyl pyrrolidone.

6.

Method according to one of claims 1 to 5, characterized in that no more than 5 minutes after the addition of the basic compound, the temperature of the mixture is at least 100 ° C.

Method according to one of claims 1 to 6, characterized in that the compound with basic effect is added during distillation. Method according to one of claims 1 to 7, characterized in that the distillation is carried out at a temperature of 100 to 300 ° C and a pressure of 0.005 to 10 bar. 9. Method according to one of claims 1 to 8, characterized in that distillation is carried out in the absence of oxygen.

10.

Method according to one of claims 1 to 9, characterized in that the method is executed continuously.

eleven.

Method according to one of claims 1 to 10, characterized in that the distillation of the N-alkyl pyrrolidone to be purified occurs in admixture with additional N-alkyl pyrrolidone from the production process.

Method according to one of claims 1 to 11, characterized in that the purified N-alkyl pyrrolidone obtained after distillation, on 100 parts by weight N-alkyl pyrrolidone, contains a total of less than 0.5 parts by weight of other components - less than 0.05 parts by weight of water and - in total less than 0.005 parts by weight of compounds of formulas I and II. 13. Method for the use, purification and new use of N-alkyl pyrrolidones, characterized in that a) a polymer is dissolved in an N-alkyl pyrrolidone, b) lithium ion accumulator electrodes are produced with this polymer and in this regard an N is recovered Contaminated alkyl pyrrolidone, which contains at least one of the contaminants of formulas I or II 10 image3 where R represents hydrogen or a C1-to C20 alkyl group, c) to the N-alkyl pyrrolidone recovered and to be purified a basic compound is added and no more than 20 minutes after the addition of the basic compound, the temperature of the mixture increases to at least 80 ° C, d ) N-alkyl pyrrolidone is distilled off from the mixture obtained, and e) is used again as a solvent for the production of lithium ion accumulator electrodes. eleven